A conventional light emitting diode (“LED”) device uses an epoxy as encapsulating material. The encapsulation process is frequently accomplished by injection molding, transfer molding or casting. Cured epoxy encapsulant has relatively high hardness, which provides resistance to scratches and abrasion, high rigidity, and high initial light transmissivity. Conventional encapsulated LED devices come in a variety of sizes and styles, such as 4 mm Oval LED Lamps, 5 mm Round LED Lamps, Chip LEDs and plastic leaded chip carriers (“PLCCs”).
However epoxy-based encapsulating materials suffer from thermal and photo degradation. Degradation is especially acute if the wavelength emitted by the LED chip is in the near the ultraviolet (“UV”) portion of the spectrum. Epoxy encapsulating material degrades when subjected to high light flux, particularly if the wavelength of the light is in the range from 200 nm to 570 nm. Degradation of the encapsulant results in increased absorption of light in the blue to green wavelengths, causing a “yellowing” effect on clear epoxy encapsulant and reduced light transmissivity through the encapsulant, which causes a significant drop in the light output of the LED device. Typically, an epoxy-based 5 mm LED lamp device's light output drops by 20% or more after 1000 hours in use, and by 50% or more after 10,000 hours in use.
FIG. 1A shows a portion of a strip 100 of semi-finished conventional LED lamps 102, 104. The LED lamps 102, 104 are attached to a leadframe 106, which is fabricated as a strip of LED lamps. LED lamps are singulated from the leadframe 106 by shearing leads 108, 110. “Singulation” means separating an LED lamp or a group of associated LED lamps from a leadframe or other substrate, such as a ceramic substrate or a printed circuit board (“PCB”) substrate.
An LED chip 114 is attached to a first substrate portion 115 that electrically couples a first terminal (not shown) of the LED chip 114 to the lead 110. In a particular embodiment, the LED chip is mounted in a reflector cup of the first substrate portion using conductive epoxy. A bond wire 112 electrically couples a second terminal (not shown) of the LED chip 114 to a second substrate portion 117. The bond wire 112, LED chip 114, first substrate portion 115, second substrate portion 117, and portions of the leads 108, 110 are encapsulated in hard, rigid encapsulant 116, such as an epoxy encapsulant. The hard, rigid encapsulant 116 protects the bond wire from being damaged when an LED lamp is sheared from the leadframe 106 by securing the first and second substrate portions so that the do not move relative to each other.
FIG. 1B shows a conventional singulated LED lamp 102. The leads 108, 110 have been cut from the lead frame (see FIG. 1A, ref. num. 106, 108, 110). One lead 108 has been cut shorter than the other lead 110 to indicate the electrical polarity of the LED chip 114. Hard, rigid encapsulant 116 secures one lead relative to the other to prevent avoid damage to the bond wire 112.
FIG. 1C shows a conventional singulated LED lamp 102 inserted and clinched into a PCB 120. In a typical automated assembly process, the leads 108, 110 of the LED lamp 102 are inserted through holes in the PCB 120 and bent to secure the LED lamp 102 in place for a subsequent soldering step. The strong, rigid epoxy encapsulant secures the leads 108, 110 from movement relative to each other, which could otherwise damage the LED chip 114 and/or the bond wire 112.
FIG. 2 shows a prior-art light source 200. An LED chip 214 is attached to a PCB substrate 201. Bond wires 212, 213 electrically couple terminals (not shown) on the LED chip 214 to terminals 208, 210 on the PCB substrate 201. The terminals 208, 210 are plated through holes that allow surface mounting of the light source 200 on a surface-mount circuit substrate. The plated through holes are plugged with a compound 211, such as solder resist, before encapsulant 216 shaped as a dome is molded over the LED chip 214 and top of the PCB substrate 201. More information on such a light source is found in U.S. Pat. No. 6,806,583.
FIG. 3 is a simplified cut-away isometric view of another prior art LED device 300. An LED chip 314 is attached to a silicon substrate member 301 (forming what is commonly known as a “chip-on-chip” assembly), which is attached to a substrate member 303. In a particular embodiment, the LED chip 314 is a high-power LED chip, that is, an LED chip operating at or above 500 mW, and the substrate member 303 is metal (e.g. a metal “slug”) that provides a heat sink to conduct heat away from the LED chip 314 through the silicon substrate member 301. A lead 308 extends from a lead support member 309, which is typically a molded polymer. A second lead is not shown in this view, but essentially extends from the lead support member opposite the lead 308.
A pre-molded thermoplastic cover 316 fits over the LED chip 314 to form a cavity 315 within the LED device 300. The cover 316 is shaped to form a lens according to the desired light intensity distribution pattern of the LED device 300. The cover forms a cavity 315 in which the LED chip 314 sits. Subsequently liquid silicone encapsulant is introduced into the cavity by dispensing or injecting it through an opening in the package to fill the entire space in the cavity 315 within the package, and the encapsulating material is then cured. The silicone filling the cavity 315 provides a soft, optically transparent material having a refractive index greater than 1.3.
Using silicone materials with high-power LED chips, particularly those operating in the blue to green portion of the spectrum, is desirable because silicone is less prone to yellowing than the epoxy used in LED lamps such as the LED lamp 102 shown in FIG. 1B. However, the packaging techniques used in the LED device 300 of FIG. 3 are relatively elaborate, involving many parts and assembly steps.
Hence, there is a need for LED devices that do not degrade like conventional LED lamps, yet do not require the number of components and assembly steps typically used to package high-power LED chips.